Vasopressin (antidiuretic hormone, ADH) in addition to the renin-angiotensin-aldosterone system, plays a major role in the regulation of blood composition and volume. Blood volume, in turn, is a key, if not predominant, factor in the long-term regulation of blood pressure. In addition, there is increasing evidence that ADH, by virtue of its pressor properties, is important in the maintenance of blood pressure following an acute reduction in blood volume. Thus, it would appear that ADH is of consequence in normal cardiovascular regulation. In that event, it is reasonable to question whether ADH is involved in such abnormalities of blood pressure regulation as hypertension, particularly in those forms of hypertension which are dependent on an expanded blood volume. On the other hand, it must also be considered that blood volume and pressure are important determinants of ADH secretion. Thus, even if ADH is not of etiologic importance in hypertension, the hypertension itself may affect ADH secretion and metabolism. We therefore propose to study the role of ADH and the interaction of ADH with the renin-angiotensin system in one kidney Goldblatt hypertension in the conscious dog and in the rat with hereditary hypothalamic diabetes insipidus. These studies should not only provide additional insight into the etiology and therapy of hypertension, but also into the physiology of ADH in normal cardiovascular regulation. Plasma levels of ADH, renin, and angiotensin II will be measured by radioimmunoassay.